There are presently in existence in the United States approximately 190,000 emergency locating transmitters (ELTs) required by law on all small aircraft traveling more than twenty-five miles from an airport and about 6,000 emergency position indicating radio beacons (EPIRBs) required on certain classes of marine vessels. ELTs and EPIRBs are essentially the same device which transmits an audio tone on 121.5 MHz and 243 MHz indicating that a distress incident has occurred. In general, the ELTs are activated by aircraft impact whereas the EPIRBs are activated in response to contact of the EPIRBs with water. The audio output generated by these devices is a characteristic distress signal which is produced by a modulation waveform having a downward sweep of at least 700 Hz within a band of 1,600 Hz and 300 Hz at a rate of 2 Hz to 4 Hz. These specifications are predetermined by legislation so that the required transmitted signals may be recognized by individuals listening for the characteristic distress signal on the 121.5 MHz and 243 MHz carrier frequencies. The Air Force Rescue Coordination Center (AFRCC), for example, receives information from others, i.e., FAA flight service stations, airports, Civil Air Patrol, etc., who monitor the 121.5 MHz and 243 MHz carrier frequencies and it responds to distress signals by notifying search and rescue personnel (SAR), such as the Civil Air Patrol, to search for the location of the distress transmission and initiate rescue operations.
While these distress signal devices have been employed for some period of time and with some degree of success, some problems have surfaced with respect to these devices in terms of their reliability. These problems can be generically divided into two categories, one category being a failure of the device to operate when operation is intended and the other relating to its operation when operation is not intended, i.e., false alarms. Furthermore, traditional, legislated ELTs and EPIRBs do not provide any indication to the pilot or other operator as to whether the device is ready for activation or, on the other hand, whether the device is activated and producing a false-alarm signal.